Radio frequency (RF) systems employing frequency conversion utilize a tunable local oscillator to scan a wide input frequency range. A frequency synthesizer is often used for this purpose to generate a local oscillator signal having an accurate and stable frequency.
A significant problem exists when the frequency synthesizer is required to have a wide tuning range and a small tuning step while maintaining low phase noise and reference spurs. For example, spurious tones in the local oscillator signal of an RF receiver can appear in intermediate frequency (IF) signals or baseband signals after mixing the local oscillator signal with a received RF signal. Spurs within the system bandwidth typically cannot be filtered with a bandpass filter and therefore the spurs can limit the spur-free dynamic range (SFDR) of the RF receiver. Phase noise in the local oscillator signal can also degrade the RF system performance.